In a typical disc brake device of a caliper floating type, a caliper is supported to a mounting bracket movably in a rotation axis direction of a wheel. In this case, a pair of slide pins are arranged at the caliper and a pair of fit holes to be fitted with the pair of the slide pins are arranged at the mounting bracket. By fitting the slide pins respectively to the fit holes as being slidable, the caliper is allowed to move. The caliper is U-shaped as striding a disc rotor. An inner pad (i.e., a friction pad) is movably supported to one side of the caliper and an outer pad (i.e., a friction pad) is fixed to the other side thereof. Further, an actuator having a piston to push the inner pad to the disc rotor is provided at one side of the disc rotor.
Accordingly, when a driver depresses a brake pedal, the actuator is activated with the depression force, so that the piston moves frontward to push the inner pad to the disc rotor and the caliper moves by reactive force due to the frontward movement of the piston so as to push the outer pad to the disc rotor. In this manner, by sandwiching the disc rotor with the pair of pads, brake force can be exerted on the wheel via the disc rotor.
By the way, in the above disc brake device, the piston is movably supported to a cylinder hole of the caliper and is maintained as being liquid-tight with a piston seal. It is configured that the piston moves frontward while deforming the piston seal when operating fluid is supplied to a hydraulic pressure chamber of the cylinder hole during braking and that the piston moves rearward to the hydraulic pressure chamber side due to restoring force of the deformed piston seal when hydraulic pressure is released from the hydraulic pressure chamber. Here, when the brake pedal depressing force by a driver is excessively large, the piston is pushed into the disc rotor side more than necessary and relative movement occurs between the piston and the piston seal. Then, so-called dragging, that is, continued contacting of the pad with the disc rotor, occurs since the piston cannot sufficiently move rearward with the restoring force of the piston seal. Accordingly, there may be occurrence that the piston is pushed back as the pad being kicked by the rotating disc rotor, that is, so-called knock back.
For example, Patent Literatures 1, 2 disclose solutions for such problems.
In a disc brake device disclosed in Patent Literature 1, an annular ring attaching groove is formed at an inner hole of a cylinder and a chamfered portion is formed to a pad side at an opening edge of the pad side of the ring attaching groove. Further, a retraction ring which seals a section between the piston and the cylinder and returns the piston during depressurization after braking is assembled to the ring attaching groove. Further, a circular recession opened to a pad side end wall of the ring attaching groove is formed at the cylinder and a deformable ring made of compressive material is assembled in the circular recession to allow entering deformation of the retraction ring into the circular recession. Here, hydraulic pressure at which deformation amount of the pad and the cylinder increasing in accordance with hydraulic pressure increase becomes larger than retraction amount of the piston caused by the retraction ring can be set at a high value. Accordingly, dragging can be suppressed even in a usage range having higher hydraulic pressure compared to the related art.
Further, in a disc brake caliper disclosed in Patent Literature 2, a piston seal frictionally engaged with an outer circumferential face of a piston due to a retainer and a disc spring urging the retainer in the direction of rearward movement of the piston are arranged at a large diameter hole portion formed at a caliper main body. Here, in the case of low brake fluid pressure, frontward movement of the retainer is blocked due to preload of the disc spring and the piston seal is elastically deformed into a chamfered portion due to frontward movement of the piston, and then, the piston is retracted by restoring force of the seal when braking is released. When a brake pad is worn, frontward movement with sliding against the piston is allowed after the seal is elastically deformed to an allowable limit, so that clearance can be corrected. On the other hand, in a case of high brake fluid pressure, the retainer and the seal move frontward as overcoming the preload of the disc spring, so that the piston is reliably retracted without occurrence of sliding against the piston. Accordingly, occurrence of dragging phenomenon of a pad can be prevented.